In semiconductor processing systems, a workpiece, such as a silicon wafer, is commonly positioned by a workpiece retaining apparatus. In some systems, a wafer is transferred from an atmospheric environment into a load lock chamber, where after evacuation of air from the load lock chamber, the wafer is transferred to an electrostatic chuck or platen for subsequent processing, such as an ion implantation. Once processing of the wafer is complete, the process is reversed and the wafer is removed from the electrostatic chuck and returned back to atmosphere via a similar load lock chamber. In some ion implantation systems, such as the Optima HD systems manufactured by Axcelis Technologies of Beverly, Mass., the wafer is further positioned on a pivoting mechanism having a load platen that is horizontal for transfers of the wafer in atmosphere, while being vertically positioned for transfers of the wafer in vacuum. In such a system, the load platen generally defines a door of load lock chamber in the vertical position.
In operation of the load platen, a transfer location is provided for the wafer to be dropped off by an atmospheric robot, whereby the wafer is positioned on pins, whereby clamps, commonly referred to as grippers, are positioned around a periphery of the wafer to hold the wafer during the transition from the horizontal to the vertical orientation via the pivoting mechanism. As such, sensing of the wafer as being properly held by the grippers is desirable in order to ensure secure wafer handling. Such sensing is typically provided in the vacuum environment of the load lock chamber to ensure proper positioning of the wafer therein.
Conventionally, the pins on which the wafer rests are adjustable to provide the proper location of the wafer for the atmospheric robot to place the wafer thereon. It is desirable, for example, for the grippers to clamp the wafer without shifting the position of the wafer, as misalignment of the wafer with respect to the grippers can cause a generation of particles that can decrease the yield of the semiconductor process. The grippers, for example, are further actuated by a gripper actuation system in a controlled manner in order to reduce chances of particle generation. Since the wafer is gripped while it is in the vacuum environment, and in-vacuum actuators are generally expensive and complicated, the gripper actuation system is typically located on the atmospheric side of assembly.
However, such a conventional system has several drawbacks. For example, in some ion implantation steps, the wafer may be cooled (e.g., to approximately −50° C.) during implantation. In order to prevent condensation on the wafer when returning the wafer to atmospheric pressure, the wafer should be near the ambient temperature (e.g., above the dew point of the atmospheric air) when the load lock is vented to expose to the atmosphere. If this is not the case, water from the atmosphere will condense on the wafer and potentially cause particles to adhere to the surface of the wafer. Thus, when the wafer is cooled during processing, the wafer is subsequently heated by a heater in the load lock chamber after the load lock chamber is vented with very dry gas (e.g., nitrogen having a low dew point). The heated gas thus warms the wafer to generally eliminate the condensation problem, but such heating can also cause electronic sensors associated with the workpiece sensing to fail when the sensors are proximate to the grippers.
For example, the heated gas is conventionally heated to a high temperature (e.g., approximately 250° C.) in order to raise the temperature of the wafer to approximately room temperature within a time period, such that the time period does not limit the throughput of wafers through the system. Accordingly, internal sensors associated sensing the gripping of the wafers within such systems commonly have a high failure rate due to the high temperatures involved, and the effect of such high temperatures on the electronic components of the sensors.
Further, conventional gripping systems often have difficulties in the placement of the grippers with respect to the wafer. For example, two opposing linear actuators are commonly used, whereby one gripper is coupled to one of the linear actuators, and two grippers are coupled to the other linear actuator, whereby a three-point contact with the wafer is achieved. Wafers are commonly misplaced by the atmospheric robot when the wafer and grippers are not adequately aligned or centered in such an arrangement. As such, mishandling and/or breakage of wafers can result from such misalignment. Furthermore, the pins on which the wafer rests typically require alignment to the grippers in order to prevent mishandling and particle generation. Accordingly, conventional systems have three support pins to adjust in height, as well three grippers to adjust in multiple dimensions, where such adjustments are commonly tedious and awkward, as well as typically requiring a high degree of skill to accomplish correctly.